The present invention relates to lubricating compositions and, in particular, to lubricating compositions containing antioxidants.
It is well known that lubricants are susceptible to deterioration due to oxidation resulting in a decline of performance. The oxidation process leads to a loss of lubricating properties and an inadequate protection of a device or machinery to be lubricated. A general description of lubricants and lubricant oxidation can be found in Kirk-Othmer's Encyclopedia of Chemical Technology, Vol. 15, pp. 463-515 (4th Edition, 1995). The oxidation of lubricants normally proceeds via a free-radical mechanism. To inhibit oxidation, various compounds are added to lubricants to serve as oxidation inhibitors. The oxidation inhibitors function by breaking the free-radical chain reactions which lead to lubricant oxidation.
An example of one type of oxidation inhibitor is non-aromatized polymers of 2,2,4-trimethyl-1,2-dihydroquinoline (known as TMDQ). The polymerization of 2,2,4-trimethyl-1,2-dihydroquinoline and the use of TMDQ polymers with non-aromatic terminal units as oxidation inhibitors are disclosed in U.S. Pat. No. 2,908,646 to Roach et al., which is herein incorporated by reference.
Despite the availability of oxidation inhibitors, such as non-aromatized TMDQ polymers, there is a continuing need for new oxidation inhibitors that provide improved performance for lubricating compositions. Accordingly, it is an object of the present invention to provide lubricating compositions with increased performance.